



V . t • . 









!>* 




> - » • • « *Vk ft> . i • o «^' 







C v ♦' 









r <v 










<?*. *..•' aP' 
















_r' * ^»' 



A_V _ ■ *. ^j 






*" G<"">. >. 



0* »• VL** "*b 



j* v . 




& °- X/<ssfc% <?*Zi&k°» /-^.\ <? <£X&: » <**-;S£i% 







<» *'TVT» .0 



X5 *o . . * /► 



o^, *<> . » • J* 



<+. ••• 



.* o 






4-""^ V 



V . « * o "^ 



^ 



Jp 



^ 



I* . t • I 



V °y> * • » 



...' ,G V 



<> *'T^ 






./\ 




» ^ • 










<> *'...' ,& 






»♦ . N 



«5°- 







.4" H 














o^ •♦. 











.*% 













^•0* 

5° 






















o , ' ^ 

v 






o > 






**F 






?• J\. ''$£&; ^ : 




'0 






> ... v^\^ ^^^v v-^y v^v \*^* 4 / ^ 

-•fife' *<U** .^ ( <JS& 1 o >.^ :dbt'. ^** SuMtir. V>* *!dfe\ ♦* .*♦ >Wa 



% 





W Sgffl-. \>S S» \S #fc \S :gS£* \S .-M- 






r.- ^ -.fw: #'%. -,u^.- ^'*, -..^^.- ^»*^ -• 








IC 8931 



Bureau of Mines Information Circular/1983 




Economic Evaluation of a Method 
To Regenerate Waste Chromic 
Acid-Sulfuric Acid Etchants 



By Deborah A. Spotts 




UNITED STATES DEPARTMENT OF THE INTERIOR 



faj&Msii*, ^ox^- y*>^) 



y 



Information Circular 8931 



Economic Evaluation of a Method 
To Regenerate Waste Chromic 
Acid-Sulfuric Acid Etchants 

By Deborah A. Spotts 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



# 



\ fc 



This publication has been cataloged as follows: 



-\\\l^ 



■ a V> 



X\0> 



Spotts, Deborah A 








Economic evaluation of a nic 


hod to re 


generate 


waste chromic add- 


sulfuric acid etchants. 








(Information circular / Bureau 


of Mines 


; 8931) 




Bibliography: p. 7. 








Supt. of Docs, no.: I 28.27:8931. 






1. Etching reagents— Recyc 


ing— Economic as 


uce ts. 2. Chromic 


acid. 3. Sulphuric acid. I. T 


itle. II. 


Series: 


Information circular 


(United States. Bureau of Mines) 


; 8931. 






TN295.U4 [TU899.M43] 


622s 


[671.71 


83-600071 



CONTENTS 

Page 



: 



Abstract 1 

Introduction 2 

Operation of electrolytic cell 2 

Economics 3 

Capital costs 3 

Operating costs 4 

Economic evaluation 4 

References 7 

ILLUSTRATIONS 

1. Daily direct operating cost 6 

Daily sludge treatment and disposal savings 6 

3 . Daily sodium dichromate savings 6 

TABLES 

1. Estimated fixed capital cost 3 

2. Estimated annual operating cost 4 







^ 



UNIT 


OF 


MEASURE ABBREVIATIONS USED IN THIS REPORT 


ft 




foot mo month 


gal 




gallon pet percent 


kW'h 




kilowatt-hour yr year 


lb 




pound 



ECONOMIC EVALUATION OF A METHOD TO REGENERATE 
WASTE CHROMIC ACID-SULFURIC ACID ETCHANTS 

By Deborah A. Spotts ] 



ABSTRACT 

Researchers at the Bureau of Mines have developed a technique for re- 
generating chromic acid-sulfuric acid etching solutions used in metal 
surface treatment operations. The technique utilizes a diaphragm cell 
equipped with a cation-selective membrane to oxidize Cr 3+ to Cr 6+ at 
the anode and to remove copper, the major metallic contaminant, at the 
cathode. 

Normally, spent etchant is discarded after approximately 3 days of 
use. Using the electrolytic cell, the etchant can be used for a year 
without replacement. From data obtained from industrial-scale cells, 
the installation of a regeneration cell with a 1,000-gal catholyte- 
holding tank has been estimated to save at least $240 per day. The 
payback period for the investment is estimated to be about 10 mo or 
less. 

Because the magnitude of these cost savings will vary at different 
locations, several graphs are presented to aid in calculating payback 
for a specific site. Using these graphs and the capital costs pre- 
sented in this study, the payback period can be determined for install- 
ing a regeneration cell with a 500- or a 1,000-gal catholyte-holding 
tank in an existing surface treatment plant. 



1 Chemist, Avondale Research Center, Bureau of Mines, Avondale, Md. 



INTRODUCTION 



In 1980, there was no domestic mine 
production of chromium in the United 
States; however, the United States con- 
tinued to be a major chromium consumer 
(1) .2 To aid conservation of the metal, 
researchers at the Bureau of Mines de- 
vised a method to regenerate chromic 
acid-sulfuric acid etching solutions, 
thereby saving the chromium lost when the 
solution is dumped. 

These solutions currently are used for 
brass and printed circuit board etching 
and other surface treatments . For these 
solutions to be effective, the chromium 
must be in the hexavalent state. As the 
solutions are used, the Cr 6+ is reduced 
to Cr 3+ ; the dissolved solids content, 
and therefore the specific gravity, of 
the solution increase. During plant 
operation, sodium dichromate is added to 
the solution to replace the reduced chro- 
mium and losses due to drag out. These 
losses increase as the specific gravity 
increases. Sodium dichromate additions 
can extend the life of the etchant some- 
what, but when the solution no longer 
performs properly, despite additions, it 
is discarded to a waste operation. Dur- 
ing treatment all the metals including 
chromium are precipitated as waste solids 



(usually hydroxides) for disposal in a 
landfill. 

The method devised by the Bureau of 
Mines, and subsequently demonstrated in a 
brass etching plant, uses a diaphragm 
cell equipped with a cation-selective 
membrane to oxidize the Cr 3+ to Cr 6+ at 
the anode and remove copper, the major 
metallic contaminant, at the cathode 
^2-4). This increases the solution life, 
reduces waste disposal costs significant- 
ly, and lowers chromium losses to drag 
out. 

Bureau of Mines researchers, after per- 
forming bench-scale and small pilot plant 
studies, designed and operated a process 
research unit at Gould, Inc., Valve and 
Fittings Division, Niles, 111.3 (now Im- 
perial Clevite, Inc.) for a 17-day peri- 
od. After evaluating the performance of 
the Bureau's cell, personnel at Gould, 
Inc., contacted Scientific Control Labo- 
ratories, Inc., Chicago, 111., and re- 
quested that a larger cell be constructed 
and installed for continuous use. This 
cell has been in operation for over a 
year without a need for etchant disposal. 
This operation has been used as an addi- 
tional source of data for this study. 



OPERATION OF ELECTROLYTIC CELL 



The electrolytic cell described herein 
is designed to continuously regenerate 
and purify chromic acid etching solu- 
tions. It is assumed that the cell is 
added to an existing surface treatment 
plant that uses chromic acid etching 
solutions. 

The electrolytic cell is located adja- 
cent to an etching tank containing a 
chromic acid-sulfuric acid etchant. This 
cell consists of one to five anode 
chambers and cathodes in a single tank 
containing a sulfuric acid catho- 
lyte. Each anode chamber consists of a 

^underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



lead-antimony electrode surrounded by a 
cation-selective Nafion membrane. This 
membrane allows the positively charged 
ions to pass but inhibits the passage of 
negatively charged ions. Test results 
indicated that this membrane is not de- 
graded by dichromate ions. The cathodes 
are sheets of copper metal, and the cath- 
olyte in the tank is a 10- to 20-pct sul- 
furic acid solution. The concentration 
of sulfuric acid in the catholyte must be 
within 5 pet of that in the spent etchant 
to minimize the transfer of water across 
the membrane. 

^Reference to specific companies, trade 
names, or manufacturers does not imply 
endorsement by the Bureau of Mines. 



Spent etchant is pumped from the etch- 
ing tank through a cartridge filter to 
remove any metal particulates. The solu- 
tion then is metered through manifolds 
into the anode chambers. The surrounding 
cation-selective membrane does not allow 
the etchant to mix with the catholyte. 
In the anode compartment, the following 
reaction occurs: 

2Cr 3+ + 7H 2 -»- Cr 2 7 2 - + 14H + + 6e - . 

The complementary reaction at the cathode 
is 

3Cu 2+ + 6e" * 3Cu. 

(These reactions are simplified versions 
of more complex reactions. The Cr 3+ and 
Cu 2+ probably exist as complex sulfate 
species.) The Cu 2+ ions migrate from the 
etchant through the membrane into the 



catholyte. Regenerated etchant is re- 
turned to the etching tank. 

Copper deposits loosely on the cathodes 
and, in one modification, slides down the 
cathodes into plastic buckets attached to 
the cathode assemblies. To remove the 
copper, the cathodes are lifted from the 
cell with a hoist approximately once a 
week, and the buckets are dumped manual- 
ly. After washing, the cathode product 
is 99 pet copper. 

In addition to the cell, some accessory 
equipment is required for operation, in- 
cluding a hoist for lifting the cathodes 
and piping between the etching tank and 
the electrolytic cell. Ductwork and a 
blower are added for ventilation of the 
cell, and electrical connections are 
needed for the cell and the blower. 



ECONOMICS 



The cost estimate in this evaluation is 
based on data from both the Bureau of 
Mines process research unit and the in- 
dustrial etchant regeneration operation. 

CAPITAL COSTS 

The capital costs presented in this 
evaluation are based on manufacturer's 
cost quotations and capacity-cost data. 
The electrolytic cell cost quotations 
were supplied by Scientific Control Labo- 
ratories, Inc. These costs include the 
rectifier, all required busing, the 
transfer pump, the cartridge filter, and 
the exhaust hood as well as the tank, 
electrodes, and membranes. Costs for 
cells with 500- and 1,000-gal catholyte- 
holding tanks are presented in table 1. 
The 1,000-gal catholyte-holding tank con- 
tains twice the anode area and therefore 
has twice the capacity of a 500-gal tank. 

The accuracy of a plant-addition esti- 
mate is very difficult to determine be- 
cause of the many variables that must be 
considered for each individual plant. In 
this study, it has been assumed that 
clear space is available for the cell and 



that the piping and electrical lines will 
be less than 200 ft long. If it is nec- 
essary to clear space for the cell or add 
additional electric service, the capital 
costs would be increased. Because the 
installed cell cost represents about 90 
pet of the capital cost, the estimated 
costs in this study should be satis- 
factory to show the value of using the 
cell. 

The capital costs presented in table 1 
include installation labor. It has been 
assumed that the cell will be deliv- 
ered to the plant site, installed, and 



TABLE 1. - Estimated fixed capital cost 



Electrolytic cell.... 

Hoist 

Blower 

Ductwork 

Electrical 

Piping 

Total 



1,000-gal 
cell 



$65,700 

3,400 

1,000 

100 

1,800 

200 



72,200 



500-gal 
cell 



$42,000 

3,400 

1,000 

100 

1,100 

200 



47,800 



Basis: Second quarter 1982 cost. 



guaranteed to be operational; therefore, 
no additional startup costs are included 
in the estimates. Additional working 
capital has not been included since it 
has been assumed to be minimal. 

OPERATING COSTS 

The estimated operating costs (table 2) 
are based on an average of 350 days of 
operation per year over the life of the 
equipment. The regeneration cell and the 
etching tank with which it is associated 
will operate intermittently. Electrical 
requirements and chromium savings data 
are therefore based on operational data 
averaged over a long period of time. The 
operating costs are divided into direct 
and fixed costs. 



Direct costs include utilities and 
plant maintenance. It has been assumed 
that no additional direct labor will be 
required to operate the electrolytic 
cell. Occasional checks are required to 
determine if the cell is functioning 
properly, and copper removal is neces- 
sary. Since these activities require a 
minimal amount of time, they can be per- 
formed by existing plant employees. 

Payroll overhead for maintenance per- 
sonnel includes vacation, sick leave, 
social security, and fringe benefits. 

Fixed costs include the cost of taxes 
(excluding income taxes), insurance, and 
depreciation. Depreciation is based on a 
straight-line 20-yr period. 



ECONOMIC EVALUATION 



Etching solution life can be signifi- 
cantly lengthened by using the electro- 
lytic regeneration technology described 
herein. Normally, spent etchant is dis- 
carded after approximately 3 days of use. 
With the use of the electrolytic cell, 
the etchant can be used for a year with- 
out replacement. This results in signif- 
icant cost savings in several areas, as 
follows : 

Better product quality control . — Per- 
formance of the regenerated " etching 



solution remains constant and is superior 
to that of the untreated etchant prior to 
dumping. This will result in a product 
with a more consistent quality and less 
of f -specif ication product. 

Reduced waste solution treatment and 
disposal costs . — Chemical and labor costs 
for waste solution treatment and costs 
for sludge haulage to a landfill will de- 
crease because only the drag out from the 
cell will need to be treated. As capital 
investments have already been made at 



TABLE 2. - Estimated annual operating cost 





1,000-gal cell 


500-gal cell 


Direct cost: 
Utilities: 


$5,500 


$2,800 




5,500 


2,800 


Plant maintenance : 


600 
600 


300 




400 




1,200 
200 


700 




100 




6,900 

700 

700 

3,600 


3,600 


Fixed cost: 


500 




500 




2 , 400 




11,900 


7,000 



existing plants for waste treatment 
facilities, no credit was considered for 
the reduced need for waste treatment. 

Reduced sodium dichromate consump- 
tion . — Since the etchant will last a year 
or more, the only sodium dichromate 
losses will be those due to drag out. 

Reduced drag out losses . — Because the 
etchant is continuously regenerated, the 
specific gravity will remain fairly con- 
stant. Without treatment, the specific 
gravity of the etchant increases. Drag 
out losses increase with increased spe- 
cific gravity; thus, the use of the cell 
will lower these losses. 

Copper byproduct recovery . — Copper met- 
al recovered can be sold as a secondary 
copper product. Only a small quantity of 
copper is recovered, so this will repre- 
sent a minor cost savings. 

An example of cost savings calculations 
is presented in the following discussion. 
From process research unit data, it has 
been determined that the electrical con- 
sumption is 3.6 kW*h per pound of sodium 
dichromate regenerated, and each pound of 
sodium dichromate regenerated reduces the 
amount of sludge generated in a waste 
treatment plant by 9.4 gal (5). A plant 



installing a cell with a 1,000-gal 
catholyte-holding tank can reduce sodium 
dichromate consumption by 100 lb per day 
and consequently reduce the waste sludge 
generated by 940 gal per day. This pro- 
vides a significant cost savings. At 
costs of 20^ per gallon for sludge treat- 
ment and disposal and 68^ per pound of 
sodium dichromate saved, this is a total 
cost savings of $256 per day. 

However, to determine the net cost sav- 
ings, the cost of operating the cell must 
be considered. If the electric power 
cost is 4.5)6 per kilowatt-hour, the oper- 
ating cost for the example plant operat- 
ing 350 days per year will be $20 per day 
(not including taxes and depreciation). 
Thus, the net savings will be $236 per 
day. 

A convenient method to measure the eco- 
nomic value of this investment is to de- 
termine the payback period. Payback pe- 
riod is defined as the time required to 
recover the original investment through 
cost savings. Most company managements 
consider 2- to 3-yr payback periods sat- 
isfactory for new plants and less than 
2 yr satisfactory for plant modifica- 
tions. A formula for calculating payback 
period is 



capital cost of the cell and related equipment m 
yearly net cost savings + depreciation 



For a 1,000-gal cell, the capital cost 
and depreciation, as found in tables 1 
and 2, are $72,200 and $3,600, respec- 
tively. The daily cost savings are mul- 
tiplied by the number of operating days 



per year to obtain the yearly net cost 
savings. Therefore, for the example 
plant operating 350 days per year, the 
payback period is 



72,200 



(350)(236) + 3,600 



= 0.84 yr (approximately 10 mo). 



With such a short payback period, in- 
stalling a regeneration cell with a 
1,000-gal catholyte-holding tank in the 
example plant would be considered a prof- 
itable investment. 

Costs for sludge treatment and dispo- 
sal, sodium dichromate, and electric 



power will vary for each individual plant 
considering installing a regeneration 
system. For this reason, graphs have 
been provided to show how variances in 
these costs affect the cost savings. 
Figure 1 shows the effect of the elec- 
tric power cost on the operating cost for 



both a 500- and a 1,000-gal cell. Fig- 
ure 2 illustrates the effect of sludge 
treatment and disposal costs on the cost 
savings for both cells, and figure 3 
shows the effect of the cost of sodium 
dichromate on the cost savings for both 
cells. Using this information, the capi- 
tal cost (table 1), and the depreciation 
(table 2), the payback period for in- 
stalling either a 500- or 1,000-gal cell 
can be calculated for a specific plant. 

The cost savings presented in this 
evaluation do not include savings due 
to the better product quality control 
because data are not available to mea- 
sure this quantity. In addition, no 
credit has been included for the copper 
metal recovered from the cell, which can 
be sold as secondary copper product. In- 
clusion of this credit will decrease the 
payback period slightly. 



1,500 



>- 

to 

XI 



o 



1 ,000 



500 




500-gal cell 



20 kO 60 80 100 120 1**0 

SLUDGE TREATMENT AND DISPOSAL COST, 
cents per gal Ion 

FIGURE 2. - Daily sludge treatment and dis- 
posal savings. 



30 



h- 


25 


co 




O 




o >- 




to 
o -a 


20 


■z. 




— 1_ 




1- 0) 




< a. 


15 


UJ if) 




O- L. 




O TO 




' — 


10 


1— — 




o O 




uj -a 




DC 





— 


1 1 


1 1 1 1 y 


— 




y/ 


— 




^n,000-gal cell 
^^-^^500-gal cell 




1 1 


1 1 1 l 



>- 

-a 



a. 



O 
■a 



C3 



100 







i i — i — i — r 




500-gal eel 1 



1 



I I I l 



50 55 60 65 70 75 80 



ELECTRIC POWER COST, 
cents per kilowatt-hour 

FIGURE 1. - Daily direct operating cost. 



SODIUM DICHROMATE COST, 
cents per pound 

FIGURE 3. - Daily sodium dichromate savings, 



REFERENCES 



1. Peterson, E. C. Chromium. BuMines 
Minerals Yearbook 1980, v. 1, p. 190. 

2. George, L. C, D. M. Soboroff, and 
A. A. Cochran. Regeneration of Waste 
Chromic Acid Etching Solutions in an 
Industrial-Scale Research Unit. Proc. 3d 
Conf . on Advanced Pollution Control for 
the Metal Finishing Industry (Kissimmee, 
Fla., Apr. 14-16, 1980). EPA-600/2-81- 
028, February 1981, pp. 33-36. 

3. Soboroff, D. M. , J. D. Troyer, and 
A. A. Cochran. Regeneration of Waste 
Metallurgical Process Liquor. U.S. Pat. 
4,337,129, June 29, 1982. 



4. Soboroff, D. M. , J. D. Troyer, and 
A. A. Cochran. Regeneration and Recy- 
cling of Waste Chromic Acid-Sulfuric Acid 
Etchants. BuMines RI 8377, 1979, 13 pp. 

5. Horter, G. L. , and L. C. George. 
Demonstration of Technology To Recycle 
Chromic Acid Etchants at Gould, Inc. 
Pres. at 4th Recycling World Congress and 
Exposition, New Orleans, La., Apr. 5-7, 
1982, 13 pp.; available from G. L. Hor- 
ter, Bureau of Mines Rolla Research Cen- 
ter, Rolla, Mo. 



INT.-BU.OF MINES, PGH., PA. 26839 



H266 83 



?»o <£><tt 










*&.«F :jw: ^u* v : 















'•■ A' 

' \* v ... V 



:• ^ ^ •.»»•' . ♦♦ * . C «W^ ^ V -W - * * . -i 



» ft* 0«.. "*1 






.A* %. *^77i* A ^ 



*W ° 






if 









A*?, 



v" ** *« °OT^ <f\ --W/ ♦♦ * "WP?-' / ? \ M 



V a* v ^ 



'• % A* •• 



W .'J 



V 



/>* v v ^ 






\0 TS * * 



-: a°* 



^^ 



? ^°- 



: > ^ -J 






^ v*^/ v?£V V^^'/ %^P*^ v^V % 



*• "^ A* *J^tjtk" \ ^ •YSfe*- ^ A* *>,Wa* ^v ^ •dSte'* U A^ »^ 









.•i^-. ^ A^..ii^^v. .v..iv-.^ 49*.%iX/.-%. v s \.iv-.^ a9 ..i^f.,% 




L - ^.^ .•jSfev ^a* ;* 



^ 



^J> .v... <&„ O* ..... •**„ .««> .v..^ ^ ft* •**« _ t 4> .w».. <>, ft* 



° A< 









' J 



: ^o„ .^ 



?- ^** '.v^-. V^. 






* A* *. 



% U A* •■ 



* .^ v * •-?»?»• > v >. -y^r^ ^ ^ -.?^®»/ > 






• 'i. A** '■JUSbA. . < 



.*«C» 






• A'O o f/^5^VVtf * eP^n ■* 



<* *•< 



^ *♦ 



^0* 




•bv" 



,' > ^ 











O > 






ft* ..... «*^ 




'.* * 



-\^ 



»1* "» C» 







A^ .«J^w'* ^ . .0 • i '.•- . *«5. a0^ . .i^L'. O 





> "* 'A 



^> 



lV >« .& , l /»i'. ^ /* »* 









» a^ ^ .; 



















X 1 



» < 



*fev* :^|^*-. *^o« . o, «^^: n,v* 



ip-^. 







^.. '••• *^ 











, ^t. 



r «. *>>»'' 9 <« 




















** A> ' .>Va« **« .<?' * % ff5^'. ' ** A* ' »>V^- "^ A"* ^ &S*. tc A* ' *', 



♦ A * 






-*■ A v * 



* «• A v ' 



fV - o " • . *V> 




. ^ 



^ 0' 

A^°^ 







*\.i^% 



V 



:, v^* .-. 



0^ »«ii- 




0^ . ° * • . %> A^ .<■'•. *^ 



°^**»^** a° V*^«° .* 



^^ 



»°^ • 






A** 



*6V 



v % »iv- ^6 a^ ,»i^:* * v .•i'i- ^ a*) \i^ * 












<, •'rvT' <g* ^> 













: ^o« 














,>* 










^•V 











/\ -JSI-' y% lf§i : /\ $8-' /% -.ip:-* s\ ; -w^* : **^ 






>v 



^^K\" "\ *\*'aJsL~% "" ^' 









0° \^^' °o ,*\0 






3SV \'W*'j? V^V V^*/ V^'V \'*^% 



^ . » • ■ 



J' A°^ \i 



•t > "*,. •.'" 










LIBRARY OF CONGRESS 



002 959 899 A 



